In recent years, liquid crystal display devices, which are image display devices, have been widely employed as display devices in computers, mobile phones, PDAs, and game consoles due to their characteristics of being power-conservative, thin, and lightweight. In general, a liquid crystal display device includes a liquid crystal panel and a backlight that illuminates the liquid crystal panel from the rear. The liquid crystal panel is configured by sandwiching a liquid crystal layer between an active matrix substrate and a counter-substrate.
The active matrix substrate is configured by forming a plurality of pixels on a glass substrate in a row-wise and column-wise matrix. Also, in the case of displaying color images, each pixel is normally configured by three sub-pixels. Each sub-pixel includes a TFT and a pixel electrode. Furthermore, the counter-substrate includes a counter-electrode and a color filter provided on a glass substrate. The color filter has red (R), green (G), and blue (B) color layers provided in one-to-one correspondence with the sub-pixels. There are also cases in which the color filter is provided on a TFT substrate.
In the liquid crystal display device, the transmittance of the liquid crystal layer is adjusted for each sub-pixel by adjusting the voltage applied between the pixel electrodes and the counter-electrode. As a result, an image is displayed in a display region of the liquid crystal panel with use of irradiated light from the backlight that has passed through the liquid crystal layer and the color layers.
In this way, conventional liquid crystal display devices include a function for displaying images, but in recent years, a liquid crystal display device also including an image detection function that can pick up an image has been proposed (e.g., see Patent Document 1). A liquid crystal display device with an image detection function such as is disclosed in Patent Document 1 includes a plurality of photodiodes formed in a matrix on an active matrix substrate, and has a position detection function for when the liquid crystal panel is used as a touch panel and an image sensor function for when the liquid crystal panel is used as a scanner.    Patent Document 1: JP 2007.072318A